(1) Field of the Invention
The present invention belongs to the field of liquid level sensors in which the liquid level within a tank is measured as a function of the capacitance between a pair of tubes when the liquid is within the region defined by the tubes.
(2) Description of Related Art
This type of sensors is well-known in the art and it can normally be grouped in two categories—that of the sensors installed at the bottom part of the tank and that in which the sensors are fixed with clamps inside the tank, being open on their top and bottom sides. The latter category presents the disadvantage of requiring the installation of electronic components, such as wiring, at the inside of the tank, that is, in the volume that can be filled with the liquid, e.g. fuel, which can give rise to problems of energy induction in the tank.
The sensors intended for being placed on a bottom surface of a tank normally comprise a probe foot by means of which the sensors are held on such surface. The sensors' electronic equipment can then be mounted within the probe foot, which is in turn electronically connected to the tubes of the sensor.
Document WO200216888 discloses an example of such sensor. A pair of upright tubes extends from a probe foot in such a way that the volume between the tubes can act as a capacitor. The outer tube comprises a series of draining holes at the region closer to the probe foot, so that the liquid can enter the capacitor volume through these holes. The bases of both of the capacitor tubes are located directly on the probe foot, as a consequence of which a trapping area is formed at the zone of the capacitor volume between the probe foot and the draining holes. If contaminants reach this trapping area, their evacuation may be difficult and may require disassembling the sensor. Moreover, this position of the bases of the tubes makes that the capacitor region starts immediately over the probe foot, as a consequence of which the presence of a droplet of water on the probe foot may result in a variation of the capacitance of the capacitor volume, and thus of the sensed liquid level.
The document U.S. Pat. No. 5,187,979 describes a probe for monitoring various attributes of a fuel, such as aviation fuel, that is contained in a storage vessel or tank. To monitor contaminants such as water in the fuel, is maintained within a vessel a conductive tank sensor member at least partially immersed in the fuel and maintaining a tank sensor member in position to form a capacitance with the fuel. A second conductive member such as a metal vessel or ground is provided for determining the magnitude of the capacitance so formed. Then, by comparing the magnitude lastly determined with a previously determined magnitude associated with substantially uncontaminated fluid, is determined the presence or absence of a contaminant in the fluid.
The document JP2010210269 describes an air bubble mixing ratio sensor for detecting such ratio with precision, in an oil level detector for an oil pan. The oil level detector includes a capacitor having a voltage applying electrode and an earth electrode. At a bottom part of the oil pan, is provided an insulating part. An electrostatic capacity calculating circuit calculates the electrostatic capacity of the capacitor on the basis of the output signal outputted corresponding to the change of a dielectric constant in the oil interposed in a diametric gap of the capacitor. A memory stores an air bubble mixing ratio map and an air bubble mixing ratio calculating circuit determines the air bubble mixing ratio on the basis of the electrostatic capacity as well as the air bubble mixing ratio map stored in the memory.
The document GB2142145 describes a liquid level sensor comprising a capacitor comprising a pair of spaced-apart plates by an insulating disk. The spaced-apart plates extend over the level range to be sensed, and a capacitance measuring device is connected to the capacitor. The capacitor plates are coaxial and an outer plate is earthed. A hollow cylinder housing the sensor is closed, but a few small holes are provided near the top and bottom of the cylinder to allow restricted access of liquid to the interior. The restricted access cuts down surge effects which might be encountered when a liquid tank is carried on a vehicle.
Other documents were considered i.e. DE9218364, DE19757924, FR1099638, U.S. Pat. No. 2,848,666 and U.S. Pat. No. 4,924,702.